Metaphors can be useful, unless they are allowed to override reality. In recent weeks, advocates for "reproductive freedom" have said that part of the Republican "war on women" is the proposal to let religious employers refuse to buy contraceptive coverage in their health insurance plans.

But who is the enemy? Most women, a New York Times/CBS News poll finds, agree that religious hospitals and universities should be free to opt out. Nearly half think any employer should have that prerogative.

If the effort to limit the contraceptive mandate were truly a frontal assault on women, a majority of them would not be endorsing the offensive. But the ideology of groups like Planned Parenthood and the National Organization for Women (NOW) sometimes ignores inconvenient gender realities.

Those advocates have been distracted from a different and far less figurative war on women -- which, as it happens, is helped rather than hindered by one of the "reproductive rights" they champion. Legal abortion may empower women, but it has also become a powerful method for the mass elimination of females.

Modern technology allows prospective parents to learn the sex of a fetus, and many of them use that knowledge to exercise a preference for sons. Absent such intervention, about 105 boys are born for every 100 girls. But as Mara Hvistendahl reports in her 2011 book "Unnatural Selection," the number for boys per 100 girls has risen to 112 in India and 121 in China.

It was once assumed that the general preference for male offspring would subside as countries became richer and women became more educated. But in country after country, that has proved false.

Nor is the phenomenon limited to the eastern hemisphere. Rajendra Kale, editor-in-chief of the Canadian Medical Association Journal, writes that "female feticide" is so common in Canada that he believes "doctors should be allowed to disclose this information only after about 30 weeks of pregnancy -- in other words, when an unquestioned abortion is all but impossible."

French demographer Christophe Guilmoto, reports Hvistendahl, regards gender imbalance as "an epidemic. In the number of lives it has touched, he says, sex selection merits comparison with AIDS." Worldwide, experts say, the number of "missing girls" amounts to a stunning 163 million -- more than the entire female population of the United States.

The gender imbalance is particularly outsized in China partly because of the government's compulsory one-child policy. Yet that policy has sometimes been excused by supporters of women's rights. In 1989, as president of NOW, Molly Yard praised the Chinese population policy as "among the most intelligent in the world."

Selective abortion, however, does not target only girls. Recent screening advances now make it easier and safer to detect Down syndrome in the womb. Universal screening will have a predictable impact, because 92 percent of fetuses diagnosed with the abnormality are aborted.

Paul Root Wolpe, director of Emory University's Center for Ethics, told the New York Post, "What you end up having is a world without people with Down syndrome."

No one would object if that were achieved by curing the condition. But eradicating it through abortion doesn't sound so benign. A survey reported in the American Journal of Medical Genetics found that only 4 percent of parents with Down syndrome children regret having them -- and nearly 99 percent of the people with the disorder said they are happy with their lives.

The practice of eliminating people who are regarded as unacceptable because of their sex or significant defects was probably an inevitable result of the proliferation of abortion. There may be others even more ominous.

A recent article in the Journal of Medical Ethics argues that abortion should not be limited to fetuses that have not yet been born. The authors propose instead to allow "after-birth abortion," which is "ethically permissible in all the circumstances where abortion would be" -- which means, really, for any reason at all.

That policy may not be so improbable. Ann Furedi, head of the British Pregnancy Advisory Service, has said, "There is nothing magical about passing through the birth canal that transforms it from a fetus into a person." The Netherlands now allows physicians to euthanize newborns with a "hopeless prognosis" and "unbearable suffering" if the parents authorize it.

Abortion-rights advocates think the right to choose has conferred great benefits. Maybe so, but not on everyone.